1. Field of the Invention
This invention relates to a laminated solid-state image pickup device, and a method for manufacturing the device, and more particularly, to a laminated solid-state image pickup device having a photoconductive film on a substrate, and a method for manufacturing the device.
2. Description of the Related Art
Recently, solid-state image pickup devices utilizing semiconductors have been increasingly used. In accordance with this tendency, solid-state image pickup devices with higher performance and lower prices have been requested.
For example, CCD's (charge-coupled devices) and MOS (metal oxide semiconductor) solid-state image pickup devices are known as solid-state image pickup devices. In many of such solid-state image pickup devices, a photosensing unit, a signal charge storage unit, and peripheral circuits, such as a signal reading circuit, a main scanning circuit, a signal processing circuit and the like, are formed in the same semiconductor substrate.
In such solid-state image pickup devices, considerably excellent characteristics are obtained in the performance of residual images, and crosstalk between pixels. However, as the device has higher definition, the photosensing area for each pixel is reduced. That is, the photosensing area per unit area, i.e., the effective numerical aperture, is reduced. As a result, the problem that it is difficult to obtain a sufficient sensitivity, in some cases, arises.
In order to provide higher sensitivity, there have been proposals of substantially increasing the numerical aperture by forming a photoconductive film on a semiconductor substrate, in which the above-described (semiconductor) circuits are formed, as a photosensing device (for example, Japanese Patent Laid-open Application (Kokai) Nos. 49-91116 (1974) and 51-10715 (1976)). A solid-state image pickup device having such a configuration is termed a laminated solid-state image pickup device.
FIG. 1 is a schematic cross-sectional view of a laminated solid-state image pickup device.
In FIG. 1, there are shown a p-type silicon substrate 1, a vertical CCD 2, an n-type cathode layer 3, trasfer gate electrodes 4, interlayer insulating films 5, a first pixel electrode 6, a second pixel electrode 7, a photoconductive film 8, and a transparent conductive film 9.
In the case of FIG. 1, a interline-transfer-type CCD image pickup device is formed on the p-type silicon substrate 1. The n-type cathode layer 3 constitutes an accumulating diode for accumulating signal charges. The vertical CCD 2, comprising an n-type buried-channel CCD, is formed close to the n-type cathode layer 3 of the accumulating diode. The first pixel electrode 6 is connected to the n-type cathode layer 3 of the accumulating diode, and the second pixel electrode 7 is connected to the first pixel electrode 6.
The transparent conductive film 9 is formed on the second pixel electrode 7 via the photoconductive film 8. The photoconductive film 8 sandwiched between the transparent conductive film 9 and the second pixel electrode 7 functions as a photoelectric conversion unit. The transfer gate electrodes 4 are made of polysilicon or the like, and transfer electric charges from the accumulating diode to the CCD channel. In order to prevent unnecessary short circuit between the electrodes and the like, interlayer insulating films 5 are provided.
As shown in FIG. 1, in a typical laminated solid-state image pickup device, the n-type cathode layer (an accumulating portion) 3, formed in the substrate 1, and the photoconductive layer 8 are connected by the pixel electrodes 6 and 7.
In the laminated solid-state image pickup device having the configuration shown in FIG. 1, the numerical aperture can be substantially 100%. Hence, the device of this configuration is advantageous over a non-laminated solid-state image pickup device from the viewpoint of an increase in the sensitivity. However, in order to realize an untrahigh-definition solid-state image pickup device having more than two-million pixels, the sensitivity must be further increased.
Furthermore, the problems that crosstalk between pixels increases because the distance between adjacent pixel electrodes is reduced, and that capacitive residual images caused by the capacitance of the photoconductive film are produced, may arise. Such problems are obstacles for providing the performance required for obtaining an image having higher picture quality.
Laminated solid-state image pickup devices which solve the above-described problems have been proposed.
For example, in order to reduce crosstalk between pixels, a proposal of providing a control electrode for preventing crosstalk between pixel electrodes is described in Japanese Patent Laid-open Application (Kokai) No. 4-30577 (1992).
In order to reduce capacitive residual images, a proposal of configuring a connecting conductor for connecting pixel electrodes to a first-conduction-type layer (for example, an n-type-semiconductor layer) of a signal charge storage diode by a first-conduction-type semiconductor, and providing a second-conduction-type layer from a side of the connecting conductor to a second-conduction-type channel stopper layer (for example, a p-type-semiconductor layer) via the surface of a first-conduction-type impurity layer of the accumulating diode is described in Japanese Patent Laid-open Application (Kokai) No. 63-66965 (1988).
A high-sensitivity and high-response-speed photoelectric transducer including a photoconductive region having a carrier multiplication function is described in European Patent Laid-open Application No. EP437633.
A laminated solid-state image pickup device, in which a photoconductive region having a carrier multiplication function is connected to an accumulating capacitive portion using a semiconductive or metallic connecting member, is described in European Patent Laid-open Application No. EP542152.
However, the above-described solid-state iamge pickup devices still have room for improvement with respect to reduction in residual-image characteristics and crosstalk between pixels.
In order to improve residial-image characteristics, it is effective to reduce capacitive residual images. In order to reduce capacitive residual images, it is desirable to completely deplete a region between the photoconductive film and the accumulating capacitive portion.
However, in the laminated solid-state image pickup devices described in Japanese Patent Laid-open Application (Kokai) No. 4-30577 (1992), and European Patent Laid-open Application Nos. EP437633 and EP542152, a photoconductive layer is electrically connected to an accumulating capacitive portion via a metallic electrode or a semiconductive layer including a high-density impurity, and there is no idea of providing a completely depleted region. Hence, there is room for solving the problem of capacitive residual images.
The laminated solid-state image pickup device described in Japanese Patent Laid-open Application (Kokai) No. 63-66965 (1988) has a schematic cross-sectional view shown in FIG. 2.
In FIG. 2, components having the same reference numerals as in FIG. 1 are the same components as those shown in FIG. 1. In FIG. 2, there are shown a p-type a-Si (amorphous silicon) layer 10, an n-type single-crystal Si whisker (a connecting member) 11, an n-type a-Si electrode (a pixel electrode) 12, an undoped a-Si layer (a photoconductive film) 13, a p-type a-SiC film 14, and a p.sup.+ -type channel-stopper layer 15.
As described above, a connecting conductor for connecting the n-type a-Si electrode 12 to the n-type cathode layer 3, serving as the first-conduction-type layer of the signal charge accumulating diode, is formed by the n-type single-crystal Si whisker 11, and the p-type a-Si layer 10 is provided at the circumferential side of the n-type single-crystal Si whisker 11. The p-type a-Si layer 10 is connected to the p.sup.+ -type channel stopper layer 15.
In the photoelectric transducer shown in FIG. 2, in order to substantially increase the numerical aperture, the n-type a-Si electrode 12 is used as the pixel electrode. The area of the n-type a-Si electrode 12 is greater than the area of the connecting portion of the connecting conductor connected to the accumulating diode. Accordingly, when the distribution of the elctric field obtained when a bias voltage is applied to the transparent conductive layer on the photoconductive layer is considered, it is very difficult to deplete the photoconductive film over the entire area of the pixel electrode, and to transport photocarriers, which have reached the pixel electrode, in the lateral direction of the pixel electrode to the accumulating capacitive portion of the accumulating diode. Hence, although residual images can be reduced, there is still room for improvement.
In the device described in Japanese Patent Laid-open Application (Kokai) No. 63-66965 (1988), the single-crystal Si whisker connecting conductor is formed after forming a CCD, using a vapor/liquid/solid-phase growth method (VLS method). In this production method, restrictions are present for the process temperature for circuitry in the substrate, and a high-temperature process for forming a connecting conductor having a low defect density cannot be used. Accordingly, in the production method described in Japanese Patent Laid-open Application (Kokai) No. 63-66965 (1988), it is difficult to sufficiently reduce defects in the connecting conductor, and there is room for improvement in residual-image characteristics.
In the laminated solid-state image pickup devices shown in FIGS. 1 and 2 and described in the foregoing patent applications, as the number of pixels per unit area increases, i.e., as the density of pixels increases, the distance between adjacent pixels is reduced, thereby causing, in some cases, the problem of crosstalk between adjacent pixels.
Also in laminated solid-state image pickup devices in which the area of the pixel electrode is substantially the same as the area of the accumulating capacitive portion of the circuitry in the substrate, sufficient characteristics cannot, in some cases, be obtained due to leakage between pixels via defects present in the interface between the photoconductive film and the insulating film.
As described above, it is difficult to simultaneously reduce capacitive residual images and crosstalk between pixels in laminated solid-state image pickup devices, and there is still room for improvement in laminated solid-state image pickup devices.